Field
The present invention relates to a wireless communication, and more particularly to a vector modulator and a transceiver including the same in the wireless communication system.
Description of Related Art
A system in which a transmitter and a receiver are simultaneously turned on has a problem that a signal from the transmitter is input to the receiver, so that receive sensitivity is degraded. An example of such a system can include a radio-frequency identification (RFID) transceiver in which the transmitter and receiver share an antenna. The RFID technology refers to a technology of wirelessly bringing information of an RFID tag to an RFID recognizer.
In general, a passive RFID tag has no separate power for driving itself. Therefore, the RFID transceiver transmits continuously an RF continuous wave (CW) signal from the transmitter of the RFID transceiver in order to supply energy to the passive RFID tag. Simultaneously with this, the receiver included in the RFID transceiver receives data from the RFID tag. By doing this, a part of the output signal of the transmitter of the RFID transceiver is input to the receiver as a leakage signal, so that the receive sensitivity of the receiver is degraded.
The following methods for removing a transmitter leakage signal which is input to the receiver of the RFID transceiver have been being applied. 1) A method is applied which uses two orthogonally disposed antennas to the transmitter and receiver respectively of the RFID transceiver. However, this method requires high cost and the size of the RFID transceiver becomes larger. 2) A method is applied in which the transmitter and receiver of the RFID transceiver share one antenna and a circulator or an isolation device like a directional coupler is used between the transmitter and the receiver. However, in spite of the use of the isolation device, due to the finite isolation rate (20˜30 dB) of the isolation device and the change of the reflection ratio of the antenna, which changes according to the external environment, the amplitude of the transmitter leakage signal which is input to the input terminal of the receiver is generally larger than 0 dBm. Therefore, the receive sensitivity of the RFID transceiver is still degraded.
There is a demand not only for the above-described method for increasing the rate at which the transmitter is isolated from the receiver outside the transceiver chip, but also for a method for additionally removing the transmitter leakage signal inside the transceiver chip. The following technologies have been applied to the method for additionally removing the transmitter leakage signal. 1) It is possible to use a phase modulator which adjusts the phase of the output signal of the transmitter by using a phase shifter and controls the amplitude of the phase-adjusted signal by using a variable amplifier, thereby outputting a signal capable of offsetting the transmitter leakage signal which is input to the receiver. The phase modulator uses, as shown in FIG. 1a, the phase shifter which adjusts the phase of the transmitting signal and the variable amplifier which controls the amplitude of the transmitting signal are used. Here, it can be considered that the order of the phase shifter and the variable amplifier is reversed. Generally, the phase shifter can adjust the phase by adjusting a signal delay through the control of the current or through the control of the magnitude of a capacitor. 2) It is possible to use a vector modulator which obtains two signals having a 90 degree phase difference from each other by using the output signal of the transmitter and then controls the amplitude of each signal and sums the signals, thereby outputting a signal capable of offsetting the transmitter leakage signal which is input to the receiver. A typical vector modulator uses, as shown in FIG. 1b, an RC-CR circuit and a polyphase filter for generating two orthogonal signals, and the variable amplifier for controlling the amplitude of the signal. 3) It is possible to use an active blocker rejection method which clamps the signal which is input to the receiver, and thus, extracts the component of the transmitter leakage signal from the input signal, so that the transmitter leakage signal is removed from the input signal. Since the active blocker rejection method extracts, as shown in FIG. 1c, the component of the transmitting signal leaked from a clamping circuit (limiter) connected in parallel with a low-noise amplifier (LNA), there is little phase difference between the extracted transmission leakage signal and the transmission leakage signal which is input to the receiver. Therefore, it is possible to remove the leaked transmitting signal only by controlling the amplitude of the signal without additionally adjusting the phase. However, a part which controls the amplitude of a replica signal through the clamping circuit consumes much current. According to the relative phase difference between a tag signal and the leakage signal, a part of the tag signal, together with the leakage signal, is removed, so that noise figure characteristic may be deteriorated. 4) It is possible to use a method for removing the transmitter leakage signal by amplifying only the envelope of the signal which is input to the receiver through use of a property in which a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is on/off controlled according to a threshold voltage Vth. This method consumes relatively small current because the amplifier is, as shown in FIG. 1d, turned on and operated on only the envelope of the receiver input signal including the tag signal and the transmitter leakage signal. However, since a part of the tag signal is removed according to the relative phase difference between the tag signal and the leaked transmitting signal, the noise figure characteristic is deteriorated due to the phase difference.
As described above, the existing method for removing the additional transmitter leakage signal has a problem of its own and includes an active device like a polar phase and/or amplifier. Therefore, there occur various problems including large currency consumption due to the use of the active device. Accordingly, there is a demand for a method which has a simple structure and minimized power consumption and removes the transmitter leakage signal by means of RFID transceiver.